Various strains of HPV, which spread through sexual contact, cause most cases of cervical cancer. Two cervical cancer vaccines have Food and Drug Administration (FDA) approval in the U.S. — Gardasil, for girls and boys, and Cervarix, for girls only. Both vaccines can prevent most cases of cervical cancer if given before a girl or woman is exposed to the virus.

In addition, both can prevent vaginal and vulvar cancer in women, and Gardasil can prevent genital warts and anal cancer in women and men. In theory, vaccinating boys against HPV might also help protect girls from the virus by possibly decreasing transmission.

The cervical cancer vaccine is recommended for girls and boys ages 11 to 12, although it can be given as early as age 9. It's important for girls and boys to receive the vaccine before they have sexual contact and are exposed to HPV. Once infected with HPV, the vaccine might not be as effective or might not work at all. Also, response to the vaccine is better at younger ages than it is at older ages.

If the three-dose series of vaccines isn't completed by ages 11 to 12, the Centers for Disease Control and Prevention (CDC) recommends that girls and women through age 26 and boys and men through age 21 receive the vaccine. However, men can receive the HPV vaccine through age 26 if desired.

Both vaccines are given as a series of three injections over a six-month period. The second dose is given one to two months after the first dose, and the third dose is given six months after the first dose.

The cervical cancer vaccine isn't recommended for pregnant women or people who are moderately or severely ill. Tell your doctor if you have any severe allergies, including an allergy to yeast or latex. Also, if you've had a life-threatening allergic reaction to any component of the vaccine or to a previous dose of the vaccine, you shouldn't get the vaccine.

Yes. It's possible that, even if you already have HPV, you could still benefit from the vaccine. However, Gardasil and Cervarix don't treat HPV infection and only protect you from specific strains of HPV to which you haven't been exposed.

Overall, the effects are usually mild. The most common side effects of both HPV vaccines include soreness at the injection site (the arm), headaches and low-grade fever. Sometimes dizziness or fainting occurs after the injection. Remaining seated for 15 minutes after the injection can reduce the risk of fainting. In addition, Cervarix might also cause nausea, vomiting, diarrhea or abdominal pain.

The CDC and the FDA continue to monitor the vaccines for unusual or severe problems.

The cervical cancer vaccine — either Gardasil or Cervarix — is part of the routine childhood vaccines schedule. Whether or not a vaccine becomes a school enrollment requirement is decided on a state-by-state basis.

HPV spreads through sexual contact. To protect yourself from HPV, use a condom every time you have sex. In addition, don't smoke. Smoking doubles the risk of cervical cancer.

To detect cervical cancer in the earliest stages, see your health care provider for regular Pap tests. Seek prompt medical attention if you notice any signs or symptoms of cervical cancer — vaginal bleeding after sex, between periods or after menopause, pelvic pain, or pain during sex.

HPV vaccine Gardasil (human papillomavirus): What you need to know. Centers for Disease Control and Prevention. http://www.cdc.gov/vaccines/hcp/vis/vis-statements/hpv-gardasil.html. Accessed July 18, 2013.

HPV vaccine Cervarix (human papillomavirus): What you need to know. Centers for Disease Control and Prevention. http://www.cdc.gov/vaccines/hcp/vis/vis-statements/hpv-cervarix.html. Accessed July 18, 2013.

Gardasil vaccine safety: Information from FDA and CDC on the safety of Gardasil vaccine. U.S. Food and Drug Administration. http://www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/VaccineSafety/ucm179549.htm. Accessed July 18, 2013.

What you need to know about cervical cancer: Symptoms. National Cancer Institute. http://www.cancer.gov/cancertopics/wyntk/cervix/page5. Accessed July 18, 2013.

Centers for Disease Control and Prevention, et al. Recommendations on the use of quadrivalent human papillomavirus vaccine in males — Advisory Committee on Immunization Practices (ACIP). MMWR Recommendations and Reports. 2011:60;1705.

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